1. Field of the Invention
The present invention relates to a method of protecting metalized surfaces from contamination and degradation due to environmental conditions and, more particularly, to a protective barrier which increases the adhesion qualities of a metalized surface for subsequent polymer casing.
2. Description of the Prior Art
The physical and chemical properties of metals have made metals useful in a variety of applications. Often, however, the use of one desirable property of a metal in a specific application may be precluded by other properties of the metal. For example, the collection of humidity and chemicals normally found in the atmosphere result in the corrosion or oxidation and degradation of exposed metal surfaces. Consequently, metals that are typically exposed to environmental conditions for prolonged periods of time require that a protective coating be applied to the exposed surface. However, the inert character of some metals reduces the ability to adhere protective coatings to metal surfaces.
Environmental degradation and adherent qualities of metals are of great concern in solar collector applications due to the utilization of mirrors and photovoltaic devices. Most commercial mirrors are silvered glass composite structures wherein a thin layer of silver is deposited on the surface of a glass substrate to reflect light. Silver is preferred because its reflectivity is significantly greater than other metals, although other metals can be used. A common commercial process for fabricating such mirrors is known as a wet chemical electroless process, wherein a thin layer of chemically reduced silver is precipitated onto a sensitized glass surface. In order to protect the silver layer from damage and degradation, a protective coating is applied to its exposed surface. A common protective coat comprises a copper electroless layer deposited over the silver layer, with a thick enamel paint spread over the copper layer. The copper layer interposed between the silver and paint layers enhances adhesion of the paint to the silver. A typical composite silvered glass mirror structure is shown in FIG. 1.
Silvered glass mirror structures as described above have been used indoors without significant problems for many years. The more recent interest in the use of mirrors for solar collector applications, such as heliostats, has resulted in more outdoor test installations of mirrors. While such tests have indicated economic feasibility of mirrored solar collector concepts from an energy standpoint, they also revealed that conventional silvered glass mirrors do not stand up very well in outdoor environments. In fact, the reflective properties of most conventional mirrors undergo substantial degradation of their reflective properties within several months to several years in outdoor environments, which is a relatively short time when a useful life of 20 years is generally considered to be a minimum design requirement for economical solar collector installations.
Photovoltaic devices or solar cells used in solar collector applications suffer from the same drawbacks that confront the outdoor use of mirrors. Solar cells have two electrodes, a metal layer affixed to a substrate and metalized grid lines on the surface opposite the metal layer of the substrate. These metal surfaces, much like silvered surface of mirrors, are also prone to environmental degradation and contamination.
Accordingly, there have been many attempts to extend both the lives of mirrors, solar cells and metal surfaces, in general, by applying protective barrier layers to the metal or by encasing the entire devices in water resistant and weather impervious encapsulants. The technical papers by Yamamoto et al. and Laibinis et al. entitled "Self-Assembled Layer of Alkanethiols on Copper for Protection Against Corrosion" J. Electrochem. Soc., 140:2, 436-443 (1993) and "Self-Assembled Monolayers of n-Alkanethiolates on Copper Are Barrier Films that Protect the Metal against Oxidation by Air", J. Am. Chem. Soc. 114: 9022-9028 (1992), respectively, report that n-alkanethiolates adsorb from solution onto copper surfaces thereby forming densely packed self-assembled monolayers that slow the oxidation of the copper surface by reaction with atmospheric dioxygen. Viventi in his U.S. Pat. No. 3,346,405 discloses the use of a silicon-bonded mercaptoalkyl radical as the active ingredient for protecting silver and copper surfaces from corrosion and attendant discoloration. U.S. Pat. No. 3,649,373 issued to Dahms describes the use of attaching various small organic thiol compounds containing aromatic rings through a sulfur/metal bond to silver surfaces for the passivation of metal surfaces that are exposed to elevated temperatures. U.S. Pat. No. Re. 24,819, issued to Murphy, discloses a polishing composition that contains long-chain alkane thiols in combination with mild polishing agents designed to clean and passivate the surface of silver articles. Tracy et al. in their U.S. Pat. No. 4,963,012 discloses the use of silicon nitride as a protective diffusion barrier for metalized mirror structures. While Workens, in his U.S. Pat. No. 4,255,214 discloses the use of a 0.25% or more solution of tolytriazole as a protective layer for the prevention of oxidative degradation of the silver and/or copper films of a mirror.
In order to provide adequate protection against degradation of the photovoltaic cells by corrosion of the electrodes, caused by potentially harmful ambient conditions Marshall in his U.S. Pat. 4,953,577 discloses a method of encapsulating a photovoltaic device in a two component fluorinated polyurethane material with 3-glycidoxy propyltrimethoxy silane. While Baudin et al., U.S. Pat. No. 4,249,958, discloses a method of manufacturing a panel comprising at least one photovoltaic cell located between a transparent sheet and a second sheet adhered together using an adhesive, polyvinyl butyryl. Louis et al. discloses adhesive compositions that contain organopolysiloxanes having SiC-bonded vinyl groups and Si-bonded hydrogen to be used for bonding laminated glass and for gluing silicon semiconductor elements onto substrates in manufacturing solar cells.
The above technical papers by Yamamoto et al. and Laibinis et al. and U.S. Patents issued to Murphy disclose the adsorbance of n-alkanethiolates to metal surfaces thereby protecting the metal surface from oxidative reactions with the environment. A disadvantage of these approaches is that the adhesive characteristics of the metal are unaffected. Consequently, while n-alkanethiols may provide protective barriers for the metal's surface they do not enhance the adherent characteristics of the metal. Therefore, n-alkanethiols are not ideal candidates in applications which require both protective and adhesive qualities. Furthermore, the abrasives present in Murphy's composition will interfere with the formation of a pin hole free molecular monolayer.
While Marshall and Baudin et al. disclose methods of protecting photovoltaic devices from degradation as a result of exposure to environmental conditions using encapsulating materials, a disadvantage is that the grid lines of the photovoltaic device are not adequately protected from environmental reaction and migration of reaction products into the encapsulants matrices. Consequently, the efficiency of the grid line as a conductor over time decreases due to corrosion build-up.
There is still a need, therefore, for a single passivating agent that possesses the ability to act not only as a protective barrier to a metal surface but also acts to increase the adhesive characteristics of the metal surface.